Conventionally, a lithium ion battery which comprises a positive electrode containing lithium cobalt oxide (LiCoO2) as an active material, a negative electrode containing graphite as an active material, and an electrolyte layer sandwiched between the positive electrode and the negative electrode and capable of conducting lithium ions is known (for example, refer to Non Patent Literature 1). Lithium ions are occluded or released (intercalation or deintercalation) by lithium cobalt oxide or graphite at the positive electrode or the negative electrode in association with charge and discharge so that the lithium ion battery functions as a battery.
However, because the lithium cobalt oxide becomes less able to maintain a crystalline structure when a lithium extraction amount is increased, usually available capacity of the lithium cobalt oxide is considered to be 120 to 140 mAh/g, although the theoretical capacity of the lithium cobalt oxide is 274 mAh/g. On the other hand, the graphite has capacity of 372 mAh/g, and has capacity approximately three times of the capacity considered usually available in the lithium cobalt oxide.
Accordingly, in the lithium ion battery, a mass of the positive electrode active material is needed to be approximately three times of a mass of the negative electrode active material when structuring the battery, and there is a problem in that an energy density per mass cannot be increased sufficiently.
In order to solve the above-described problem, a lithium ion oxygen battery which comprises a positive electrode containing oxygen as an active material and lithium oxide or lithium peroxide, a negative electrode containing graphite as an active material, and an electrolyte layer sandwiched between the positive electrode and the negative electrode and capable of conducting lithium ions has been proposed (for example, refer to Patent Literature 1). In the lithium ion oxygen battery, the positive electrode comprises an activated carbon fiber and is open to the atmosphere, and oxygen in the atmosphere is oxidized by the activated carbon fiber.
In the lithium ion oxygen battery, at the time of discharge, as shown in the following formula, metallic lithium occluded (intercalation) by the graphite is ionized to generate lithium ions and electrons at the negative electrode. The generated lithium ions are released (deintercalation) from the graphite and migrate through the electrolyte layer to the positive electrode.
On the other hand, oxygen taken up from the atmosphere receives electrons to become oxygen ions, and the oxygen ions react with the lithium ions to generate lithium oxide or lithium peroxide at the positive electrode. Electric energy can be extracted by connecting the negative electrode and the positive electrode with a conductive wire.
(negative electrode) 4Li→4Li+4e−
(positive electrode) O2+4e−→2O2−                4Li++2O2−→2Li2O        2Li++2O2−→Li2O2         
In addition, at the time of charge, as shown in the following formulas, lithium ions, electrons, and oxygen are generated from lithium oxide or lithium peroxide at the positive electrode, and the generated lithium ions permeate the electrolyte layer to be transferred to the negative electrode. The lithium ions receive the electrons to be precipitated on the negative electrode as metallic lithium. The precipitated metallic lithium is occluded (intercalation) by the graphite.
(positive electrode) 2Li2O→4Li++O2+4e−                Li2O2→2Li++O2+4e−        
(negative electrode) 4Li++4e−→4Li
According to the lithium ion oxygen battery, since the positive electrode uses oxygen in the atmosphere as an active material, the mass of the positive electrode active material is not subjected to the restriction, and an energy density per mass can be increased.